1. Field of the Invention
The present invention relates to a method for a wireless communication system and a communication device thereof, and more particularly, to a method for handling cell change in the wireless communication system, and the communication device thereof.
2. Description of the Prior Art
Third Generation (3G) mobile communication technologies employ Wideband Code Division Multiple Access (WCDMA) to provide a high spectrum utilization efficiency, wide coverage and high quality, high speed multimedia data transmission. In the 3G mobile communication system, a user may use wireless communication devices (e.g. mobile phone) to perform real time video communication, Conference Calls, real time gaming, music streaming, and to send/receive E-mail, etc. However, these functionalities depend on a fast and real time data transmission. Therefore, to enhance WCDMA technology, the prior art provides a High Speed Packet Access (HSPA) system, including High Speed Downlink Package Access (HSDPA) technology and High Speed Uplink Package Access (HSUPA) technology, to increase bandwidth utilization efficiency and processing rates of packet data, to improve uplink and downlink transmission speeds.
Please refer to FIG. 1, which is a schematic diagram of a conventional HSDPA system. The Radio Network Controller (RNC) 10 may connect to multiple base stations (Node Bs) 12, and communicate with the base station 12 via an lub interface 16. The base station 12 may transmit data from the RNC 10 to a User Equipment (UE) 14. In the UE 14, a physical layer may receive physical data from the base station 12 via a High Speed Physical Downlink Shared Channel (HS-PDSCH) 18, and return (upload) information such as Acknowledgement/Negative Acknowledgement (ACK/NACK) and Channel Quality Identifier (CQI) via a High Speed Physical Control Channel (HS-DPCCH) 15. On the other hand, in the base station 12, a medium access control layer utilizes a medium access control high speed entity (MAC-hs Entity) to manage functionalities such as packet scheduling, data rate adaptation and Hybrid Automatic Repeat Request (HARQ), etc.
Please refer to FIG. 2, which is a sequence diagram of a cell change procedure with Radio Resource Control (RRC) signaling according to the prior art. The UE connects to an HSDPA cell A, to receive services provided by the cell A (e.g. mobile originating call or mobile terminating call services). When the UE moves from the cell A to another HSDPA cell B (i.e. the UE detects the strength of a signal from the cell A is gradually decreasing, whereas the strength of a signal from the cell B is increasing), and detects that signal strengths from the cell B reach a certain level, the UE transmits a measurement report 1a to the RNC, and the RNC initiates a cell addition procedure corresponding to the cell B, according to the measurement report 1a. During the cell addition procedure, the RNC transmits an active cell update message to the UE, to add the cell B to an active set of the UE, wherein the active set is a set of cells communicating with the UE. After adding the cell B to its active set, the UE responds with an active cell update complete message to the RNC. Next, the RNC transmits a Physical channel Reconfiguration message to the UE, to establish a physical channel corresponding to the cell B. After completing physical channel establishment, the UE responds with a Physical channel Reconfiguration complete message to the RNC. Finally, the RNC transmits a Measurement control message to the UE, to update a neighbor list information in the UE.
After completion of the cell addition procedure corresponding to the cell B, the UE transmits a measurement report 1d to the RNC, to request a handover from the cell A to the cell B, to change a serving cell of the UE. Therefore, after receiving the measurement report 1d from the UE, the RNC transmits a physical channel reconfiguration message to the UE, to reconfigure a physical channel of the UE to a physical channel corresponding to the cell B. After completion of the physical channel reconfiguration, the UE transmits a physical channel reconfiguration complete message to the RNC to complete the serving cell change. Note that, changing of a serving cell is implemented via a handover procedure, as will be familiar to those with general skills in the art, and is not iterated here.
Furthermore, after completing serving cell change, the UE transmits a measurement report 1b, to request the RNC to initiate a cell removal procedure corresponding to the cell A. During the cell removal procedure, the UE receives an active cell update message from the RNC, to remove the cell A from its active set. After removing the cell A from its active set, the UE transmits an active cell update complete message to the RNC. Next, the RNC transmits a physical channel reconfiguration message, to release a physical channel between the UE and the cell A. After receiving the physical channel reconfiguration complete message, the RNC transmits a measurement control message to the UE, to update the neighbor list information of the UE.
As mentioned above, serving cell change of the UE is implemented via a handover procedure. When cells A and B belong to different base stations, the handover procedure performed by the UE is an inter Node B handover procedure. Moreover, the handover procedure can be a hard handover or a soft handover. In an HSDPA system, data over the HS-PDSCH is managed via the MAC-hs Entity in the base station; therefore, an HSDPA system only supports hard handover. In more detail, when the UE performs the cell change procedure via hard handover, each base station independently manages the HS-PDCH. Thus, during hard handover, the UE does not concurrently receive a same data transmitted from a serving base station and a target base station, but only receives data transmitted from the target base station. Conversely, during soft handover, the UE may concurrently receive data from the serving base station and the target base station.
Further, in an HSDPA system, the base stations have a buffer to store data transmit from the RNC to the UE. As mentioned above, since an HSDPA system does not support soft handover, when the UE performs serving cell change via hard handover, the UE can only receive data from the target Node B. However, the original serving base station may still have remaining data to transmit to the UE. As such, data loss or data re-transmission may occur, thus prolonging data transmission time.
Moreover, when the UE performs the cell change procedure via hard handover, discontinuation of data transmission occurs. Please refer to FIG. 3, which is a schematic diagram of a relationship between transmission throughput and time during conventional hard handover procedure. As shown in FIG. 3, during hard handover, data transmission throughput over the HS-PDSCH is 0, and data transmission delay caused by the hard handover is about 300 ms, severely undermining real time services for the UE, e.g. Voice over Internet Protocol (VoIP) or Video Telephony over IP, and lowers communication quality.